1. Field of the Invention
The present invention relates to a reflective liquid crystal display device incorporated in image display apparatuses and a projection display apparatus using the reflective liquid crystal display device, and is in particular, to the device and apparatus which are able to display high-contrast images.
2. Description of the Related Art
In recent years, projection display apparatuses with liquid crystal display devices have been grown into popular tools that can be used as large-sized screen display apparatuses. Such display apparatuses, which can be used for various things, such as meetings with screen presentation, home theaters, and other uses, have been developed on various types of display devices. One type of such display apparatuses is a display apparatus with a reflective liquid crystal display device incorporated therein.
This reflective liquid crystal display device has two substrates and a liquid crystal layer, where one substrate has a surface with a transparent electrode formed thereon and the other substrate has a surface on which reflective electrodes and drive circuits for respective pixels are arranged in a matrix and both surfaces of the two substrates are opposed in parallel to each other to sandwich the liquid crystal therebetween as a layer. In this display device, the drive circuits for the liquid crystal can be embedded beneath the display pixels, so that the display pixels allow the projection display apparatus to present images with high spatial resolution and high brightness.
The liquid crystal display device utilizes the double refraction of the liquid crystal molecules to control the transmission of light passing therethrough. This means that the way the liquid crystal molecules are oriented has a large influence on the display quality of the images. Taking this fact into account, a variety of types of orientation ways for liquid crystal, which are on different operation modes, have also been studied and proposed for the reflective liquid crystal display device. One proposal is provided by “Shin-Tson Wu and Deng-Ke Yang,”“Reflective Liquid Crystal Displays,” JOHN WILEY & SONS, Ltd, published Jan. 1, 2001.” This reference provides each orientation technique (i.e., relationship among an oscillation direction of incident polarized light, incident-side liquid crystal orientation and pixel-side liquid crystal orientation) based on HFE (Hybrid Field Effect) mode, MTN (Mixed Twisted Nematic) mode, SCTN (Self-Compensated Twisted Nematic) mode and ECB (Electrically Controlled Birefringence) mode (refer to FIGS. 31A-31D of this application). In addition, Japanese Patent No. 2616014 and United State Patent Publication No. 2004/0165128 also provide other orientation techniques, which are shown in FIGS. 31E and 31F, respectively.
In the case of the orientation techniques on the above operation modes, a vertically aligned type of liquid crystal (on homeotropic aligned mode) provides high contrast and operates faster in the response speed than a horizontally aligned type of liquid crystal (on homogeneous aligned mode). Because of these features, this liquid crystal has attracted attention. In the horizontally aligned type of liquid crystal, the liquid crystal molecules are aligned substantially horizontally to the substrate surface when no voltage is applied between the substrates, whilst when a voltage is applied, the liquid crystal molecules align vertically to the substrate surface in response to its dielectric anisotropy. These alignments of the liquid crystal molecules produce black and white representations. However, though the liquid crystal molecules are ordered to be aligned vertically, some molecules positioned close to an orientation membrane formed on each substrate are held at angles which are near to the horizontal angle. This causes differences in phase of the molecules, thus deteriorating the level of black (e.g., deteriorating contrast in the black and while levels).
In contrast, the vertically aligned type of liquid crystal has a negative dielectric anisotropy, so that the liquid crystal molecules are aligned to a direction perpendicular to the substrate surface when no voltage is applied between the substrates. And, in response to an application of voltage, the molecules are made to be aligned horizontally along the substrate surface, providing high contrast and operating at a faster speed, with still less power consumption.
However, the vertically aligned type of liquid crystal still has some difficulties, which include disclination, for instance. To be specific, unless the liquid crystal molecules are respectively given a slight tilt (pre-tilt angle) in a certain direction in a state where there is no voltage application, respective molecules are flipped toward different directions, which is called disclination, thus causing a deterioration in image quality. As shown in FIG. 32, for example, the pre-tilt angle is given as an angle θp made between a long-axis direction of a liquid crystal molecule and the substrate surface in a state where no voltage is applied (meanwhile, the pre-tilt angle may be given as an angle θp′ (=90 degrees−θp), which advances from the normal line to the substrate surface, as illustrated in FIG. 32.)
In FIG. 32, there is shown an azimuthal angle α made between an azimuth produced by projecting the long axis to the substrate surface and a predetermined axis on the substrate. A difference between the azimuth angles on both the upper and lower substrates (on the light incidence side and the pixel side) corresponds to a twist angle. For example, United State Patent Publication No. 2004/0165128 shows an orientation for liquid crystal, in which a pre-tilt angle θp is 75-88 degrees (in this reference, since the pre-tilt angle is given as an angle which advances from the normal line to the substrate, the pre-tilt angle is denoted as 2-15 degrees) and a twist angle φ is 90 degrees, as shown in FIG. 31F.
By the way, in using the reflective liquid crystal display device, giving the pre-tilt angle θp results in lowering the contrast of images to be displayed. That is, for realizing higher contrast, it is desired that the vertically aligned type of liquid crystal be used. In this liquid crystal, a pre-tilt angle θp is given to the molecules to prevent the disclination, but such a previous tilt-angle setting will cause shifts in phases of the molecules, which will invite deterioration in contrast of images.
One normal countermeasure against the above difficulty is to employ a phase compensator to compensate the phase shifts for accomplishing high contrast. This countermeasure is true of a structure where a polarizing beam splitter of either MacNeille type or wire grid type is used as a polarizing device. For the phase compensation, it is required for the phase compensator to have an A-component (a phase difference) having refractive anisotropy along the surface thereof.
On the other hand, in manufacturing the reflective liquid crystal display device, there arise irregularities (errors) in both the thickness of a liquid crystal layer and pre-tilt angles to be given, device by device, no matter what the precision for manufacturing is made higher. That is, such irregularities are inevitable. Therefore, an A-component (i.e., a phase difference caused due to differences in pre-tilt angles and thicknesses of liquid crystal layers) to be compensated varies depending on each device, so that it is almost impossible to use a phase compensator of which refractive anisotropy is set to an ideal value.
In addition, the liquid crystal has a refraction index depending on a wavelength dispersion characteristic and the refraction index itself has anisotropy also depending on the wavelength dispersion characteristic. Thus, for example, the shorter the wave length, the larger the anisotropy of the refraction index. Accordingly, it is necessarily required for the wavelength plate to have the capability of compensating the phase shifts at a degree which is decided by taking the wavelength dispersion characteristics into account. Necessarily, this way will lead to a narrower selection of materials for the phase compensator. This is another disadvantage.